When creating a client with clnt_tli_create, it uses strdup to copy
strings for these fields if nconf is passed in. clnt_dg_destroy frees
these strings already. Make sure clnt_vc_destroy frees them in the same
way.
This change matches the reference (OpenSolaris) implementation.
Tested by: David Wolfskill
Obtained from: Bull GNU/Linux NFSv4 Project (libtirpc)
MFC after: 2 weeks